1. Field of the Invention
The present invention relates to satellite constellations and more particularly to providing high precision location information between satellites.
2. Description of the Related Art
Satellites are often flown in so-called constellations to assure that they can remain in constant communication with large areas of the earth""s surface. When orbiting in this fashion, the satellites spend part of their time sensing each other""s locations and attitudes. As a result, there is substantial background art associated with constellation positioning. However, this art relates to systems that use a relatively low degree of precision.
A much greater number of applications would exist for constellations if the satellites"" positions could be ascertained to a finer level of precision than presently provided. For example, optical imaging satellites have largely reached their limits in their ability to spatially resolve features. Higher resolutions will only be possible by increasing the size of their optical apertures. Systems are sufficiently large that improvements of only a factor of 2-3 are realistically possible over present architectures.
Using a coherent array of detectors can solve this dilemma. An analogous system is used in the microwave or radiowave regime. The ability to resolve small distant objects had reached its limit because increasing the size of the dishes had become impractical. By using large arrays of radio telescopes, for the example, the very large array or VLA, a large synthetic aperture can be generated having a resolving power that is equivalent to the diameter of the array. The VLA dishes extend across 36 kilometers. Even though the individual dishes are only 25 meters in diameter, they operate coherently. Consequently, they are able to produce a resolving power that is the equivalent of a single dish with a 36 kilometer aperture, without the commensurate construction difficulties.
In order to allow the dishes to work as a unit, the signals that they receive must be analyzed in a coherent fashion. Calibrations that are performed can take advantage of the fact that, to a high degree of precision, the receivers do not move since they are firmly rooted on the earth""s surface. This advantage is not present for orbiting satellites.
What is required for the satellites"" free movement is a device that can provide a high precision distance reference so that all the satellites in a space borne array know exactly where they, and all the other satellites, are located. Optical wavelengths are on the order of 10xe2x88x926 meters, while satellite separations may be 105 meters. The implication is that a reference standard would need to be accurate to 1 part in 1012, at a minimum.
In U.S. Pat. No. 4,700,150, J. L. Hall et. al. discuss a means to stabilize a laser frequency. The ""150 system incorporates an acousto-optic frequency shifter and a fast electro-optic phase modulator. The system operates on a single laser frequency that is stabilized by referencing its frequency to an external stabilized source. It does not involve the production of multiple frequencies. Therefore, accuracy is limited.
In U.S. Pat. No. 6,142,423, J. W. Wehner discusses ephemeris and attitude determination by using the optical payload installed on a satellite for pointing information. Many satellites use optical payloads for pointing and tracking. Consequently, these systems can, in some limited way, also provide information regarding satellite ephemeris. However, since the systems are not specifically designed for this, their accuracy is limited and is based upon repeated, that is, averaged measurements. Therefore, accuracy is limited. Furthermore, this system does not involve the use of laser pulses that are phase-coherent.
In U.S. Pat. No. 6,195,044 B1, R. A. Fowell also discusses the use of laser payloads, installed for optical crosslinks, for attitude determination. The ""044 architecture does not have a dedicated position sensor. Furthermore, the optical pulses are produced by a source that is not capable of producing phase-coherent pulses.
In U.S. Pat. No. 6,281,853 B1, G. Caille et. al. discuss an antenna for receiving and transmitting signals used to control the location of satellites within a constellation. However, the device exclusively uses radio wave signals. The relatively low frequencies associated with radio wave signals places a fundamental limitation on the accuracy of the position information they can provide. There is no discussion of the use of optical wavelength devices for control of the satellites.
In U.S. Pat. No. 6,295,019 B1, J. L Richards et. al. disclose another radio wave device for position sensing. The device performs its distance determination by measuring the round trip time for a signal to propagate from one source to another and back again. Consequently, the measurement is actually a two step process that measures round trip time. There is no discussion of the use of optical frequencies in this patent.
In U.S. Pat. No. 6,430,416 B1, P. V. W. Loomis discusses a satellite location system which combines GPS signals with local radio wave information. Again, there is no discussion of the use of optical wavelength radiation for controls.
Recently, optical synthesizer technology has progressed to the point where pulse timing standards routinely have accuracy of 10xe2x88x9214 and better. This is accomplished with the use of a large number of optical frequencies that are generated in the synthesizer.
As will be disclosed below in detail, the present invention uses the high precision timing afforded by pulses from an optical synthesizer to provide equivalent positioning information of sufficient resolution that it is possible to stabilize an array of spaceborne assets. Specifically, it addresses the use of an optical synthesizer to generate an extremely precise time reference that translates into an extremely precise distance reference.
In order to translate a time standard to a distance standard, maintaining the accuracy of the time standard, a velocity needs to be associated with the time, having a precision that must also be very high. Fortunately, the speed of light in a vacuum represents just such a velocity standard. It can be used to transfer the time standard""s high precision to a distance standard""s high precision.
The present invention involves the incorporation of optical synthesizers into spaceborne assets that need to be operated as an array, as a means to provide the high degree of distance precision necessary for the assets to work in a coherent fashion.
In Physical Review Letters 82 p.3568-3571, xe2x80x9cAbsolute Optical Frequency Measurement of the Cesium D1 Line with a Mode-Locked Laser,xe2x80x9d Th. Udem et. al. demonstrate how the constriction of mode-locked laser pulses to operate in a phase-coherent fashion can create a means to perform high-precision spectroscopy. The optical system disclosed may be considered to be technically close to an optical synthesizer. There is no discussion of the use of these pulses for satellite constellation positioning information.
In Physical Review Letters 84, p 3232-3235, xe2x80x9cPhase Coherent Vacuum-Ultraviolet to Radio Frequency Comparison with a Mode-Locked Laser.xe2x80x9d J. Reichert et. al. discuss the operation of an optical synthesizer for producing phase-coherent optical pulses that extend in frequency from radio frequencies to the ultra-violet. The resulting system provides a frequency measurement that is accurate to one part in 1013. However, there is no discussion of the equivalence of this frequency measurement to making accurate distance measurements. There is no discussion of satellite positioning applications.
In Physical Review Letters 84, p. 5102-5105, xe2x80x9cDirect Link between Microwave and Optical Frequencies with a 300 THz Femtosecond Laser Comb,xe2x80x9d S. A. Diddams et. al. discuss the performance enhancement of an optical synthesizer which includes a microstructure optical fiber.
In Physical Review Letters 85, p. 740-743, xe2x80x9cControlling the Phase Evolution of Few-Cycle Light Pulses,xe2x80x9d A. Apolonski et. al. discuss a means to add precision to the optical synthesizer described by J. Reichert. There is no discussion of applications.
In Physical Review Letters 85, p. 2264-2267, xe2x80x9cOptical Frequency Synthesizer for Precision Spectroscopy,xe2x80x9d R. Holzwarth et. al. discuss the highest precision optical frequency synthesizer to date. That system demonstrates precision to a few parts in 1015. The optical synthesizer generates 106 phase-coherent optical frequencies. The application mentioned in this paper relates to optical clocks. There is no discussion of the satellite constellation control.
In U.S. Pat. No. 5,079,444, R. Kallenbach et. al. discuss a nonlinear optical means to couple the interaction of an optical beam and a microwave beam. Although this patent discloses elements of an optical synthesizer, it is not related to potential satellite constellation applications.
In U.S. Pat. No. 6,038,055, T. Hansch et. al. disclose a means for producing phase-coherent pulses. The system provides amplification of phase-coherent pulses produced in a separate medium. The reference does not discuss the oscillator necessary to produce the phase-coherent pulses in the first place. Furthermore, there is no mention of the use of the pulses as a means to monitor or potentially control the position of a satellite within a constellation.
The present invention is a satellite location determination system for a constellation of satellites. The satellite location determination system includes a plurality of optical transceivers. Each optical transceiver is associated with a respective satellite. Each optical transceiver includes an optical synthesizer, transmission conditioning optics, a telescope assembly, reception conditioning optics, and an optical to electronic signal converter. The optical synthesizer produces phase-coherent optical laser pulses. The transmission conditioning optics appropriately conditions the characteristics of the phase-coherent optical laser pulses. The telescope assembly receives an output of the transmission conditioning optics and transmits an optical transceiver output into space for reception by an optical transceiver associated with another of the satellites. The telescope assembly also receives an optical transceiver output from another of the satellites. Reception conditioning optics appropriately conditions the characteristics of the optical transceiver output received from another of the satellites. An optical to electronic signal converter receives an output from the reception conditioning optics and provides an electronic signal output representing the location of other satellites in the constellation. Use of the optical synthesizer for producing the phase-coherent optical laser pulses provides enhanced satellite location information.
Use of the high precision timing afforded by the pulses from the optical synthesizer provide equivalent positioning information of sufficient resolution that it is possible to stabilize this array of satellites. It generates an extremely precise time reference that translates into an extremely precise distance reference. The optical synthesizer is capable of generating a large number (106) of optical frequencies.
Other objects, advantages, and novel features will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.